Review on Electronic and Magnetic Properties of Fe–Based Full-Heusler Compounds

Navdeep Kaur1, Vipul Srivastava1,*

1Department of Physics, School of Chemical Engineering and Physical Sciences, Lovely Professional University, Phagwara- 144411, Punjab, India Abstract

Fascinating properties of Heusler alloys made these materials remarkably imperative for researchers. In this paper review on

Fe-based full-Heusler compounds have been done. Electronic and magnetic properties of Fe2TiAl, Fe2VAl, Fe2CrAl, Fe2MnAl,

Fe2CoAl and Fe2NiAl Heusler compounds have been reviewed. All calculations were based on FP-LAPW method implemented in wien2k code. Energy and correlation potential have been estimated by using GGA approximation for Fe2YAl

(Y=Ti, V, Cr, Mn, Ni) compounds while GGA+U approximation has also been used for Fe2CoAl compound. Fe2YAl (Y=Ti,

V, Co, Ni) Heusler compounds showed metallic behavior whilst Fe2CrAl and Fe2MnAl showed half-metallic behavior because of the energy gap in the spin down case. The magnetic moment for Fe2YAl (Y=Cr, Mn, Co, Ni) was obtained as 1 µB, 2.0013 1

µB, 4.90 1 µB, and 4.28319 1 µB respectively.

Keywords: Heusler compounds, Electronic properties, Magnetic properties.

Introduction

The materials formed by the composition of three elements are known as Heusler compounds and their name was given after the name of Fritz Heusler. He is the one who first invented these materials in 1903. Since then because of their surprising magnetic behavior research on these materials is increasing day by day. Cu2MnAl was the first Heusler compound, which showed ferromagnetic behavior despite the fact of no magnetic element was present in its composition [1, 2]. Therefore these materials become proficient aspirants for memory devices, thermoelectric materials, spintronics, spin-value generators & magnetoresistive materials [3-6]. X, Y and Z elements in Heusler compounds are composite to form full (X2YZ) and half (XYZ) Heusler compounds, where X and Y are transition metals and Z is an s, p element [7, 8]. Full-Heusler compounds crystallize in L21 and XA structure [9]. Cu2MnAl

(L21) and Hg2CuTi (XA) are two crystal type structures for full-Heusler compounds which lie under the space group number 225 known as Fm-3m and 216 known as F-43m respectively. For Fm-3m (SG) X, Y and Z positioned at (0.25, 0.25, 0.25), (0, 0, 0) and (0.5, 0.5, 0.5) in an order. In F-43m (SG) X contains two sites (0, 0, 0) and (0.25,

0.25, 0.25) designated as X1 and X2 while (0.5, 0.5, 0.5) and (0.75, 0.75, 0.75) are the atomic positions for Y and Z respectively [10]. Half-Heusler compounds crystallize in C1b structure. First half-Heusler compound investigated as a half-metallic compound was NiMnSb [11].

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(a) (b)

Fig.1: (a) is representing Hg2CuTi structure and (b) is representing Cu2MnAl type structure.

Half-metallicity is another unique property of Heusler compound, in which one spin has zero gaps at Fermi energy

(EF) while other spin shows insulating or semiconducting band gap. Therefore half-metallic Heusler compounds have 100% spin polarization. This property make them imperative candidate for spintronics devices, for instance magnetic tunnel junction (MTJ) or spin valves [12]. In this paper Fe-based Heusler compounds have been studied, which are investigated by using FP-LAPW [13] method that rely upon density functional theory (DFT) [14] and implemented in wien2k code [15]. Fe-based HCs attract great interest of researchers because there are number of compounds contain semiconducting or half-metallic nature.

Electronic properties

Electronic properties play very imperative role in predicting the half-metallic ferromagnetism. The behavior of electronic states near the Fermi level reveals the nature of compound. If these are crossing the Fermi level then compound has metallic nature or gap between them describe the insulating or semiconducting nature of compound. Attractive modulus, low density, fine environmental resistance, high thermal conductivity and melting temperature of Fe2TiAl make it proficient aspirant for aeronautics and astronautics [16]. The calculated lattice parameter for

Fe2TiAl was 5.7788 Å in Fm-3m SG. Band structure of this compound confirmed the metallic behavior [17]. Some bizarre properties of Fe2VAl were noticed [18]. Other studies suggested its non-magnetic behavior but contain super para-magnetic clusters and magnetic anti-site defects [19, 20]. The optimized lattice parameter for this compound was 5.6831 Å. Both spins for Fe2VAl showed the same behavior not including any energy gap. Hence this did not show any ferromagnetic behavior and found to be nonmagnetic compound [17]. Fe2CrAl show half-metallic behavior as density of states in minority spin showed pseudogap near the Fermi level. The equilibrium lattice constant for Fe2CrAl was 5.610 Å. The electronic states shifted toward the higher binding energy in a majority spin

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© 2018 IJRAR November 2018, Volume 5, Issue 4 www.ijrar.org (E-ISSN 2348-1269, P- ISSN 2349-5138) which results in a pseudo-gap at the Fermi level [17]. The above three HCs have also been investigated experimentally [21]. In Fe2YAl (Y=Ti, V, Cr) HCs Al (s) density of states were found to be in the energy range -8 eV to -6 eV and Al (p), Fe (d) & Y (d) were responsible for the lowest scattered states. Sharma et al. showed that electronic, magnetic and optical properties of compounds got change by means of atomic configuration of HCs [17].

Fe2MnAl and Fe2NiAl investigated by F Dahmane et al. by considering Fm-3m and F-43m space group by using optimized lattice parameter 5.64 Å in Fm-3m and 5.65 Å in F-43m SG for Fe2MnAl and 5.68 Å in Fm-3m and 5.67

Å in F-43m SG for Fe2NiAl. The compound with Mn as Y found to be half-metallic in the Fm-3m SG and metallic in the F-43m SG. Band gap of 0.49 eV in the electronic states of minority spin validated the half-metallic character of Fe2MnAl. Fe (d) and Mn (d) are the partial density of states responsible for the semiconducting behavior. The compound with Ni as Y was found to be metallic in both the cases as electronic states were crossing the Fermi level

[22]. Investigation of Fe2CoAl was done by considering L21 and XA phases and found to be stable in XA phase. Different magnetic phases were also taken into account, for instance paramagnetic, ferromagnetic and antiferromagnetic phase from which ferromagnetic phase occurred as a stable phase. The compound showed metallic behavior with generalized gradient approximation (GGA) in both phases but showed a small energy gap of 0.87 eV in minority spin but at 0.5 eV above the Fermi level with GGA+U approximation, where Hubbard parameter (U) was set to 4.35 eV and 4.22 eV for Fe and Co respectively, which results in a metallic behavior of

Fe2CoAl [23].

Table 1: Observed Electronic properties and net magnetic moment.

Compound name Electronic property Magnetic moment

Fe2TiAl Metallic -

Fe2VAl Metallic 0 µB

Fe2CrAl Half-metallic 1 µB

Fe2MnAl Half-metallic 2.0013 µB

Fe2CoAl Metallic 4.90 µB

Fe2NiAl Metallic 4.28319 µB

Magnetic Properties

The calculated magnetic moment should also be reliable with the Slater-Pauling rule. It suggests that total magnetic moment should be equal to Nv-24, where Nv is number of valence electrons [24, 25]. The obtained value of magnetic moment for Fe2VAl was 0 µB, whereas 1µB for Fe2CrAl which were reliable with the Slater-Pauling rule

[17]. Fe2MnAl has 26 valence electrons which led to the integer value (2 µB) of magnetic moment and were reliable with the obtained value which is 2.0013 µB. In both Hg2CuTi and Cu2MnAl structures, Fe2NiAl had no integer value

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of magnetic moment which signified it to be metal [22]. The total magnetic moment of Fe2CoAl was not consistent with SP rule and was not an integer [23].

Conclusion

Six Heusler compounds have been reviewed and two of them found to be half-metallic whilst other four found to be

metallic. Fe2TiAl & Fe2VAl were stable in Cu2MnAl structure and were metallic in nature. Fe2CrAl & Fe2MnAl

were half-metallic in nature and achieved magnetic moment for them was 1 µB and 2.0013 µB respectively. These

also were stable in Cu2MnAl structure. Fe2CoAl and Fe2NiAl were metallic in nature and values of magnetic

moment were 4.90 µB and 4.28319 µB in a row, whereas former is stable in Hg2CuTi structure and latter in Cu2MnAl structure.

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